Red line on back of penis?

I'm 25 years old and I've had a red line on the back of my penis for as long as I can remember. I can remember having this line even from when I was in my early teens. I thought it might have been a stretch mark, but it never faded. The line is found on the shaft, and therefore I can only see it when I turn it around... or when I have an erection and I am lying on my back. What could this be? I'm concerned that is has gotten slightly bigger over the years, but I just haven't noticed it getting bigger because I haven't been paying much attention to it and the changes have taken place over many years. There is no pain or discomfort. The line is a darkish purple/brown, and thins out as it approaches the head of my penis. I'm not circumsized, and I'm pretty sure the line is found on the foreskin too, and might even be connected to the urethra.. but I'm not sure. I've tried looking around for information on this, but I can't seem to find any... Can anyone help me out here?

It's an interesting question but nothing to worry about. It almost appears as though it were a seam, right? I've got the same thing and never thought about it until your question appeared. I suspect that it's probably true with all guys.

It's called the raphe. If you search for that term you will find more about it. It's a relic of when you were an embryo and your genitals were forming in the womb. Early on the sex of the embryo is not discernable visually. There is a structure that can develop into a clitoris or a penis and a hole for the vagina. Once the true male sexual organs start to form the vaginal hole closes up leaving the raphe. It runs up from the scrotum to the tip of the foreskin and joins into the frenulum, the Y-shaped structure attaching the foreskin to the v-shaped groove under the pee hole or meatus. Every male has a raphe but those of circumcised guys end at the circumcision scar. Some of them have also lost their frenulum but this happens almost solely in the USA.
I hope that explains things,
Geoff